Positron emission tomography (PE) is a non-invasive medical imaging technique which provides an image of a positron emitting radio pharmaceutical and its distribution in the body on a metabolic level. At the present time, the performance of this instrument is limited by the nuclear sensors used. The effort proposed here will examine an approach to improve the resolution in the imaging detector of PET systems by combining the signals of two types of sensors. A photomultiplier tube is the primary detector. In addition, one or more compact photodetectors are used to obtain better spatial resolution. The goal of Phase I of this program is to demonstrate the feasibility of using silicon drift photodiodes as the semiconductor photodetector to make this approach work. If successful, the use of silicon drift photodetectors in this dual-sensor approach promises to be the breakthrough needed for the construction of a new generation of high resolution PET imaging systems.